1. Field of the Invention
The present invention relates to an electric leak detector for a vehicle, and more particularly, to an electric leak detector for a vehicle, which is configured to detect an electric leakage from a device mounted in a vehicle.
2. Description of the Related Art
In recent years, environmentally friendly vehicles, which have a battery mounted thereon and are configured to run with a motor used as a drive power source, have been developed. The environmentally friendly vehicles include an electric vehicle and a hybrid electric vehicle. In this type of vehicle, for example, a DC 12-V low voltage battery and a high voltage battery such as a DC 400-V battery pack are used in combination. As used herein, a low voltage refers to a voltage that is relatively lower than a high voltage, and herein, the low voltage has a range of from 0 V to 30 V, for example, and the high voltage has a range of from 100 V to 1,000 V, for example. A negative terminal of the low voltage battery is connected to a vehicle body, but the high voltage battery and a high voltage electric load to be fed and driven by the high voltage battery are insulated from the vehicle body. In the following, the high voltage battery and the high voltage electric load are collectively referred to as “high voltage device”.
For such high voltage device insulated from the vehicle body, a leakage resistance detection device to be fed and driven by the low voltage battery is used to measure an insulation resistance (i.e., leakage resistance) of the high voltage device so as to detect the presence/absence of ground abnormality (see, for example, Japanese Patent No. 5474114 and Japanese Patent Application Laid-open No. 2013-140074).
The leakage resistance detection device disclosed in Japanese Patent No. 5474114 includes a coupling capacitor that has one terminal connected to a high voltage device and the other terminal connected to a repetitive signal output circuit. The coupling capacitor is charged and discharged by a switching operation of the repetitive signal output circuit. In Japanese Patent No. 5474114, a leakage resistance of the high voltage device is measured via the coupling capacitor, and the measured leakage resistance is compared with a leakage determination threshold so as to thereby perform leakage determination.
In general, however, the leakage resistance value may change depending on a change in atmospheric temperature of the coupling capacitor. In Japanese Patent No. 5474114, since no consideration is taken for a rise in atmospheric temperature of the coupling capacitor, there is a possibility that a leakage cannot be properly detected and an erroneous determination occurs.
Thus, in Japanese Patent Application Laid-open No. 2013-140074, it is proposed to set a leakage determination threshold before leakage detection is performed in consideration of the rise in atmospheric temperature of the coupling capacitor and the deterioration of elements. Specifically, in Japanese Patent Application Laid-open No. 2013-140074, before the leakage detection is performed, a crest value of an AC signal is detected under a state in which a reference impedance circuit is connected, and the detected crest value is set as a reference crest value. After that, the reference impedance circuit is disconnected, and the leakage determination is performed based on the set reference crest value.
As described above, in Japanese Patent No. 5474114, no consideration is taken for the rise in atmospheric temperature of the coupling capacitor, resulting in a problem in that a leakage may be erroneously determined.
In Japanese Patent Application Laid-open No. 2013-140074, the leakage determination threshold is set before leakage detection is performed in consideration of the rise in atmospheric temperature of the coupling capacitor and the deterioration of elements. However, the configuration disclosed in Japanese Patent Application Laid-open No. 2013-140074 cannot update the leakage determination threshold in the case where the atmospheric temperature of the coupling capacitor changes during the execution of leakage detection determination. As a result, there is a problem in that a leakage may be erroneously determined.